1. Field of the Invention
The present invention relates to a high-frequency circuit board and a method of producing the same.
2. Discussion of Related Art
FIGS. 5 and 6 are flowcharts showing a conventional process for producing a circuit board, e.g. a printed-wiring board. As a blank board, a copper-clad laminate having a copper foil bonded to both sides of an insulating substrate of glass-reinforced epoxy resin, fluorocarbon resin or the like is used. After the surfaces (both sides) of the copper-clad laminate have been subjected to exposure and development, etching is carried out to remove unwanted portions of the copper foil, thereby forming copper foil patterns. After a resist has been printed on the surfaces of the copper-clad laminate, the resist on the copper foil patterns is removed, and an electrically conductive metal, e.g. gold, is deposited on the exposed copper foil patterns by plating or vacuum evaporation, thereby forming the desired patterns.
FIG. 3 shows a conventional circuit board 20 produced by the above-described process. Copper foil patterns 22 are formed on both sides of an insulating substrate 21, and coating layers 23 of an electrically conductive metal are deposited on the copper foil patterns 22. In this case, the thickness of each copper foil pattern 22 is of the order of 18 xcexcm, and each coating layer 23 of electrically conductive metal is formed as a thin-wall layer with a thickness of about 1 xcexcm or less than 1 xcexcm.
FIG. 4 is an enlarged view of part B in FIG. 3. Because the copper-clad laminate is subjected to etching, the remaining copper foil pattern 22 has sloped sidewalls. That is, the width of the copper foil pattern 22 is the largest at the bottom, i.e. at the insulating substrate side end thereof, and the width decreases gradually with distance from the insulating substrate 21. On this copper foil pattern 22, the coating layer 23 with the above-described thickness is deposited.
Incidentally, in high-frequency regions, particularly in millimeter wave or higher frequency regions, the pattern accuracy of a microstrip line affects the impedance characteristics of the transmission line to a considerable extent. Accordingly, efforts are being made to increase the dimensional accuracy of a mask used in the exposure process in order to improve the pattern accuracy of the microstrip line.
However, in the conventional circuit board, copper foil with a thickness of about 18 xcexcm is provided on the insulating substrate 21. Because the copper foil is thick, there is a difference in pattern width between the top and the bottom of the microstrip line, as shown in FIG. 4. Consequently, the transmission impedance of the microstrip line varies. Accordingly, it is difficult to obtain the desired characteristics.
The present invention was made in view of the above-described problems associated with the prior art. Accordingly, an object of the present invention is to provide a high-frequency circuit board free from variations in transmission impedance of the microstrip line and hence capable of providing the desired characteristics.
Another object of the present invention is to provide a method of producing the above-described high-frequency circuit board.
To attain the above-described objects, the present invention provides a high-frequency circuit board including a resin substrate having a surface activated to form a roughened surface. A thin-wall pattern of an electrically conductive metal is formed directly on the roughened surface of the resin substrate.
According to the present invention, a pattern of an electrically conductive metal is formed directly on a surface of a resin substrate. Consequently, no thick copper foil intervenes between the electrically conductive metal pattern and the resin substrate. Therefore, there is no variation in transmission impedance due to copper foil. In addition, the electrically conductive metal pattern is a thin-wall layer in which there is substantially no difference in width between the top and the bottom thereof. Therefore, there is substantially no variation in transmission impedance, and the desired characteristics can be obtained.
In the above-described high-frequency circuit board, the resin substrate may be made of a fluorocarbon resin material.
Accordingly, it is possible to obtain the desired characteristics even when a fluorocarbon resin material is used as a resin substrate.
In addition, the present invention provides a method of producing a high-frequency circuit board. According to this method, a surface of a resin substrate is activated to form a roughened surface, and a thin-wall pattern of an electrically conductive metal is formed directly on the roughened surface of the resin substrate.
Thus, according to the present invention, the surface of the resin substrate is roughened. Therefore, a pattern of an electrically conductive metal can be surely deposited on the surface of the resin substrate. The pattern on the high-frequency circuit board according to the present invention is unrelated to copper foil and formed in the shape of a thin-wall layer having no thick copper foil. Therefore, there is no variation in transmission impedance due to copper foil, and the desired characteristics can be obtained.
In addition, the present invention provides a method of producing a high-frequency circuit board. According to this method, a resin substrate having copper foil provided on a surface thereof is prepared. The copper foil is removed from the resin substrate to expose the surface thereof, and a thin-wall pattern of an electrically conductive metal is formed directly on the exposed surface of the resin substrate.
The surface of the resin substrate exposed by removing the copper foil is in the form of a roughened surface and hence allows a pattern to be formed thereon directly. Therefore, the activation step can be omitted, advantageously. In addition, a pattern of an electrically conductive metal can be surely deposited on the surface of the resin substrate because the copper foil has been removed therefrom. The pattern on the high-frequency circuit board according to the present invention is unrelated to the copper foil and formed in the shape of a thin-wall layer having no thick copper foil. Therefore, there is no variation in transmission impedance due to copper foil, and the desired characteristics can be obtained.
In the above-described high-frequency circuit board producing methods, the thin-wall pattern may be formed by evaporating an electrically conductive metal onto the whole surface of the resin substrate and then etching the electrically conductive metal to form the desired pattern.
By doing so, an extremely thin microstrip line having a thickness of the order of 1 xcexcm can be formed on the surface of the resin substrate. Thus, the desired characteristics can be obtained.
In the above-described high-frequency circuit board producing methods, the thin-wall pattern may be formed by covering the surface of the resin substrate with a mask formed with a pattern to be transferred and then selectively evaporating an electrically conductive metal onto the surface of the resin substrate through the mask.
With the above-described method, an electrically conductive metal can be selectively evaporated in the desired pattern on the surface of the resin substrate by using, for example, a metal mask with a pattern to be transferred that is provided as a hollow pattern. Accordingly, the step of etching the electrically conductive metal can be omitted.
In the above-described high-frequency circuit board producing methods, the resin substrate may be a fluorocarbon resin substrate.